Two weeks after the Boston Marathon bombing, Massachusetts voters will head to the polls on Tuesday to select nominees for the Senate special election.

Whether Democrats nominate Rep. Edward J. Markey or Rep. Stephen F. Lynch, the party is heavily favored to hold the seat of former Sen. John Kerry, who was appointed secretary of state.

Three Republicans — businessman Gabriel Gomez, former U.S. Attorney Mike Sullivan and state Rep. Dan Winslow — are vying for their party’s nomination. While the race changed following the bombing, few observers see a Republican able to pull off a surprise upset in the general election on June 25.

Here are five things to watch for during Tuesday’s primaries:

1. How the bombing affected the race
The special election garnered very little national attention before the bombing. But the tragedy drew the eyes of the nation to Boston, and it’s unclear how that will affect turnout or the issues that voters have in the front of their minds as they enter the voting booths.

Markey entered the final weeks of the campaign as the favorite, with a fundraising edge, a comfortable lead in the polls and establishment support, including from the Democratic Senatorial Campaign Committee. He also got a last-minute endorsement from Caroline Kennedy. Former GOP Sen. Scott P. Brown’s decision not to run sucked the drama out of what could have been a competitive general election.

Despite suspending campaign activities for a week, both Democrats landed television interviews and remained visible in the immediate aftermath. All five candidates, including the Republicans, attended the interfaith service where President Barack Obama spoke.

In the final week of the race, the Democrats have launched intensive get-out-the-vote efforts, held two debates (that featured plenty of discussion on homeland security) and have both returned to the television airwaves. Lynch immediately released a direct-to-camera ad about the bombing, and he attempted to differentiate the two candidates’ national security records during the debates. Markey also launched an ad discussing his efforts following the 9/11 terrorist attack.

2. Polls show Markey's lead varies
It’s difficult to discern how much Lynch has cut into Markey’s earlier lead because public polling has been all over the map. Markey led by around 30 points in two separate polls in the early months, while other surveys had him up by around 10 points.

This week, the Lynch campaign leaked an internal poll that showed him within 6 points of Markey. It’s rarely a good sign when a candidate boasts a poll that shows him behind, but the Lynch campaign hoped to show that the margin would continue to close.

The next day, the League of Conservation Voters released a poll showing Markey ahead by 14 points. Still, the Markey campaign is taking no chances, launching a major TV ad blitz. According to a source tracking ad buys, the Markey campaign laid down a $600,000 buy for the final week of the campaign. The three ads airing over the final days include a 60-second spot on 9/11 and two highlighting Markey's liberal record on the environment, gun laws and abortion rights.

3. Blue-collar towns are key for turnout operations
To win, Lynch will need a strong showing in his South Boston-based district. But he’ll also need strong showings in the blue-collar and working-class precincts in Markey’s district. That includes Markey’s hometown of Malden, plus Medford, Framingham and Worcester. A lot of attention has also been paid to Springfield in western Massachusetts.

By all accounts, Markey is running a champion ground game. He’s getting plenty of help in that department from the LCV, which through a couple different arms has dumped several hundred thousand dollars on a field program.

Lynch spent last week meeting with seniors and high propensity voters, while focusing field volunteers on undecided voters. The campaign knows it needs a top-notch field effort in the final days to cut into Markey’s lead and pull out a victory.

4. The Democrats are political opposites in their party
Markey is one of the most liberal members of the House. Lynch, who opposes abortion rights, is without a doubt the most conservative member of the state’s all-Democratic congressional delegation.

The difference in their records was never more apparent than in the 111th Congress, just after President Barack Obama took office. Markey was co-author of the cap-and-trade bill, while Lynch voted against the health care reform law that was widely supported by the party.

5. Markey has not run in a competitive race in a long time

The last special election primary Lynch was involved in fell on the fateful day of Sept. 11, 2001. Running for an open House seat, Lynch took a 10-point victory that day to win the Democratic nomination, and three weeks later won the heavily Democratic seat by a wide margin.

Markey, the dean of the state delegation and one of the most senior members of the entire House, hasn’t run a competitive campaign in more than 30 years. He won a 12-candidate primary with just 22 percent of the vote and since then has been held below 70 percent just twice.